There's a setting for that in the gamesetup, under "more options". But I think cheats are disabled by default in multiplayer games.
No, rams have the same lag. But you may have problems with your units trying to regroup in formation. Then they sometimes run back to the main group, which may be an issue if the main group is stuck after some obstacle. You may want to put your units in "no formation" mode to prevent this from happening. Apart from that, rams do indeed a lot of damage against buildings, so once a ram reaches a building, it's already too late.
I think that's intentional. There were players postponing their loss by building roman siege walls everywhere, and just garrisoning units in them at random places. Now that the walls decay, such a strategy isn't possible anymore (or at least harder)
Phase III is the phase that allows you to actually bring down cities. So the phase that gives you siege weaponry. If the army camps become available before that, they should have disabled siege units while in phase II at least. But I think army camps in phase II would be overpowered, as players don't have the means to bring down a building while they're in phase II.
Ranks are disabled for now, to help with balancing. But this depends on how much stronger that higher-rank units actually are. If they're marginally stronger, it doesn't make sense to alter the gui for them. If the difference is big, that info might indeed be important. This doesn't sound so difficult, so if you can program a few lines, and want to help out, it's a good place to start I guess.
That count is hard to explain, and can even depend on the structure. For now, AFAIK, all infantry and also ranged cavalry increases the arrow count by a constant, and buildings also have a default arrow count. If you select a building, you can hoover the "attack" icon (the sword and shield in the middle of the selection pane), there you can see how many arrows are fired per time, and how strong the arrows are. The tooltip is updated when you garrison or ungarrison units. So with a bit of trial and error, you can find out a lot.
Thanks